RFR: 8210064: ZGC: Introduce ZConcurrentRootsIterator for scanning a subset of strong IN_NATIVE roots concurrently

[Erik Österlund]

Hi,

We now have enough load barriers to support scanning of CLDs and JNI 
handles concurrently. I propose to do that and move these root sets out 
from ZRootsIterator, and hence the GC pause. They will be scanned during 
concurrent marking (and heap iteration), but no longer during relocation.

I still perform ClassLoaderDataGraph::clear_claimed_marks() in the pause 
because it seems cheap. But it can be moved out of the pause when Coleen 
gets her new cool CLDG lock in.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8210064/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210064

Tested through hs-tier1-3.

Thanks,
/Erik
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